Invented by English mathematician Robert Recorde in 1557, with this rationale: "I will settle as I doe often in woorke use, a paire of paralleles, or Gmowe [i.e., twin] lines of one length, thus : , bicause noe 2 thynges, can be more equalle."

His equal signs were about five times as long as the current ones, and it took more than a century for his sign to be accepted over its rival: a strange curly symbol invented by Descartes.